At 4, DS loved the Magic School Bus chapter books. The font is a decent size and there are pictures every few pages.
He also read several of the Junie B. books (like mentioned earlier) and the Stink series (Junie B's little brother). These too have enough pictures to keep kids entertained.
Too bad, the Magic Tree House series was popular with our DS at age 4!
It took a while and a number of those step readers before DS actually had the endurance to read a chapter book. He had the reading ability long before he had the endurance/patience. Good luck!
